Geographic isolation plays a major role in biological diversification. Yet, adaptive divergence also can occur with ongoing gene flow, but the minimal spatial scale required for this is unclear. Here, we hypothesized that local gradients in soil clay and water contents respectively select for anti-adherent and hypoxia-tolerant phenotypes in soil invertebrates, thereby driving intraspecific phenotypic divergence despite unlimited, passive dispersal. We tested this idea using the parthenogenetic oribatid mite Rostrozetes ovulum, an abundant species in tropical forest soils. We obtained 40 individuals from valleys and uplands within 4 km² of rainforest in central Amazonia, and estimated soil clay and water contents for each site. Then, we experimentally assessed submersion tolerance of each individual, measured its body size, shape and structural traits, and inferred anti-adherence from the extent of debris attached to its body. We found that morphological distance was greater between than within habitats while being independent of geographic distance, which itself was unrelated to habitat. Further, using structural equation modelling, we found that clayish soils harboured mites with fewer, larger dorsal pits that were less likely to have attached debris, consistent with an anti-adherent morphology. To a lower degree, individuals from moister soils tended to survive submersion longer, likely through anaerobiosis. These patterns could reflect phenotypic plasticity, local adaptation or some combination thereof. Altogether, they suggest that environmental gradients may trigger local-scale animal diversification in soils, contributing to the exceptional biodiversity of this substrate.

地理隔离在生物多样化中起着重要作用。然而，随着基因的不断流动，适应性分歧也可能发生，但是尚不清楚为此所需的最小空间尺度。在这里，我们假设土壤黏土和含水量的局部梯度分别选择了无脊椎动物的抗粘附和耐缺氧表型，从而尽管无限制，被动地分散，却驱动种内表型发散。我们使用孤雌生殖的蝙蝠螨Rostrozetes ovulum（热带森林土壤中的丰富物种）测试了这一想法。我们从4 km ²内的山谷和高地中获得了40个人亚马逊中部的雨林，估算每个地点的土壤黏土和水含量。然后，我们通过实验评估每个人的潜水耐受性，测量其身体大小，形状和结构特征，并从附着在其身体上的碎屑程度推断出其抗粘连性。我们发现形态学距离大于栖息地内部，而与地理距离无关，而地理距离本身与栖息地无关。此外，使用结构方程模型，我们发现粘土土壤中的螨虫数量较少，背坑较大，不太可能附着碎屑，这与反粘附的形态一致。在较低的程度上，来自潮湿土壤的个体倾向于通过厌氧菌存活更长的时间。这些模式可能反映了表型可塑性，局部适应或其某种组合。总体而言，他们认为环境梯度可能会触发土壤中局部动物的多样化，从而导致该底物的特殊生物多样性。